Pollution control devices are employed on motor vehicles to control atmospheric pollution. Such devices include a pollution control element. Exemplary pollution control devices include catalytic converters and diesel particulate filters or traps. Catalytic converters typically contain a ceramic monolithic structure having walls that support the catalyst. The catalyst typically oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in engine exhaust gases to control atmospheric pollution. The monolithic structure may also be made of metal. Diesel particulate filters or traps typically include wall flow filters that are often honeycombed monolithic structures made, for example, from porous ceramic materials. The filters typically remove soot and other exhaust particulate from the engine exhaust gases. Each of these devices has a housing (typically made of a metal like stainless steel) that holds the pollution control element. Monolithic pollution control elements, are often described by their wall thickness and the number of openings or cells per square inch (cpsi). In the early 1970s, ceramic monolithic pollution control elements with a wall thickness of 12 mil (304 micrometer) and a cell density of 300 cpsi (47 cells per cm2) were common (“300/12 monoliths”).
As emission laws become more stringent, wall thicknesses have decreased as a way of increasing geometric surface area, decreasing heat capacity and decreasing pressure drop of the monolith. The standard has progressed to 900/2 monoliths. With their thin walls, ceramic monolithic structures are fragile and susceptible to vibration or shock damage and breakage. The damaging forces may come from rough handling or dropping during the assembly of the pollution control device, from engine vibration or from travel over rough roads. The ceramic monoliths are also subject to damage due to high thermal shock, such as from contact with road spray.
The ceramic monoliths have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. For instance, the gap between the peripheral wall of the metal housing and the monolith may start at about 4 mm, and may increase a total of about 0.33 mm as the engine heats the catalytic converter monolithic element from 25° C. to a maximum operating temperature of about 900° C. At the same time, the metallic housing increases from a temperature of about 25° C. to about 530° C. Even though the metallic housing undergoes a smaller temperature change, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling typically occurs hundreds or thousands of times during the life of the vehicle.
To avoid damage to the ceramic monoliths from road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monoliths and the metal housings (thereby bypassing the catalyst), mounting mats are disposed between the ceramic monoliths and the metal housings. The process of placing the monolith within the housing is also called canning and includes such steps as wrapping a sheet of mat material around the monolith, inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing.
Typically, the mounting mat materials include inorganic fibers, optionally intumescent materials, organic binders, fillers and other adjuvants. Known mat materials, used for mounting a monolith in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), U.S. Pat. No. 5,242,871 (Hashimoto et al.), U.S. Pat. No. 3,001,571 (Hatch), U.S. Pat. No. 5,385,873 (MacNeil), and U.S. Pat. No. 5,207,989 (MacNeil), GB 1,522,646 (Wood), published Aug. 23, 1978, Japanese Kokai No.: J.P. Sho. 58-13683, published Jan. 26, 1983 (i.e., Pat Appln Publn No. J.P. Hei. 2-43786 and Appln No. J.P. Sho. 56-1 12413), and Japanese Kokai No.: J.P. Sho. 56-85012, published Jul. 10, 1981 (i.e., Pat. Appln No. Sho. 54-168541). Mounting materials should remain very resilient at a full range of operating temperatures over a prolonged period of use.
A need exists for a mounting system which is sufficiently resilient and compressible to accommodate the changing gap between the monolith and the metal housing over a wide range of operating temperatures and a large number of thermal cycles. While the state of the art mounting materials have their own utilities and advantages, there remains an ongoing need to improve mounting materials for use in pollution control devices. Additionally, one of the primary concerns in forming the mounting mat is balancing between the cost of the materials and performance attributes. It is desirable to provide such a high quality mounting system at the lowest possible cost.
A particular need exists to provide a mounting mat or system that provides an improved holding pressure at ambient temperature as well as at the operating temperatures to which the pollution control device that includes the mounting mat may be exposed to. It is furthermore particularly desired to find a way to improve the holding pressure in intumescent as well as in non-intumescent mounting mats. Furthermore, it would be desired to find a solution that can be used even when the mounting mat is free of organic binder or low in organic binder content. To find such a solution is particularly desirable as the use of large amounts of organic binder in a mounting mat is undesirable as it may lower the performance of the mounting mat and/or is environmentally disadvantageous as the binder usually needs to be burnt out during first use of the pollution control device. Developing mounting mats that are low in binder content and that have an improved holding pressure has proven to be particularly challenging for intumescent mounting mats because these mats are prone to cracking during the mounting of the mat in the pollution control device.
US 2006/008395 (Ten Eyck et al.) discloses a surface treatment of inorganic fibers and in particular of leached glass fibers to achieve an increased holding performance of non-intumescent mats. However, the method of manufacturing disclosed to obtain the coated fibers is cumbersome and expensive as it involves the use of a slurry of colloidal material for treating the fibers. The treatment of the fibers further includes a heat treatment which means an increased energy cost in the manufacturing.
Accordingly, it would be a further desire to find a method of making mounting mats of improved performance and that can be manufactured in an easy and convenient way at a lower cost.